The invention is related to a sifter for sifting granular material. The sifter comprises an entry for separating air and an entry for the material to be sifted, and also comprises a discharge for separating air and fine material. Also included is a coarse material discharge for the discharge of grits. The invention also relates to the grinding system in which such a sifter is to be incorporated. In the operation of high-pressure roller presses for compressing or for the pressure treatment of granular material, for example, of what is referred to as interparticle crushing of brittle material such as cement clinker. European Patent No. 0 084 383 discloses the charging stock supplied to the nip which stock must be seized by the oppositely driven rollers and drawn into the nip by friction. The individual particles of the grinding stock drawn into the nip by friction are thereby mutually crushed in a product bed, i.e., in a material fill that is compressed between the two roller surfaces with the application of extremely high pressure. The capability of the rollers to draw product in would be inherently improved if the charging stock were to be delivered in such a quantity that an abundance of charging stock is available in the charging shaft above the nip.
In the development of the art of pressing, such as pressing brittle granular material in the preparation of stock for cement making, a development was conceived which is known by the general terms of product bed comminution or interparticle crushing. In accordance with the concept, unique high pressure roller presses are arranged to apply a pressure in the nip wherein interparticle crushing occurs and the charging stock is drawn into the nip between opposed rollers and subjected to extremely high pressure such that the particles are subject to an interparticle crushing action causing incipient cracks in the particles so that they are subject to further refining or breaking up in mills such as a ball mill. This development is disclosed, for example, in U.S. Pat. No. 4,703,897, Beisner et al and U.S. Pat. No. 4,357,287, Schoenert. As used herein, the reference to interparticle crushing or product bed comminution is a reference to the art of pressing as disclosed in the foregoing patents.
It is also known to arrange such a high-pressure roller press for interparticle crushing of granular material ahead of a circulating grinding system comprising a tube mill and sifter to at least partially recirculate the grits of the sifter to the product delivery of the roller press. Particularly with high filling levels and low fill weights of the charging stock supplied to the charging shaft of the roller press and intensified by great quantities of already relatively fine grits that are recirculated to the roller press, the problem arises that the air pressed from the charging stock fill due to the high-pressure pressing can no longer escape. This is inhibiting to the material pressing and the energy saving that can be inherently achieved with interparticle crushing and leads to a non-uniform operation of the roller press. The aeration problem in the nip is further aggravated in that the fill density of the recirculated grits is usually considerably lower than the fill density of the fresh charging stock.
The object of the invention is to create a sifter that is suitable for being incorporated into a circulating grinding system that comprises a high-pressure roller press, a tube mill and a sifter in such a way that the energy-consuming aeration and material pressing difficulties in the nip of the roller press that are set forth above are considerably reduced. The invention, moreover, diminishes wear of the sifter, particularly when it is a turbo-air separator.
Characteristic of the sifter of the invention is that it comprises at least two grits discharge openings separated from one another, namely a discharge opening for coarse grits and a discharge opening for finer grits. For example, the grain size of the coarse grits can amount to more than 0.5 mm and the grain size of the finer grits can, for example, amount to less than 0.5 mm. When such a new sifter is inserted into a circulating grinding system comprising a high-pressure roller press and tube mill, then it becomes possible to recirculate the coarse grits of the sifter to the product delivery of the high-pressure roller press and recirculate the finer grits of the same sifter to the product delivery of the tube mill. The high-pressure roller press is thereby relieved of the fine material grits of the sifter, as a result of which the aeration and material pressing difficulties in the nip of the roller press are considerably diminished. The operation of the roller press becomes more uniform and the energy saving connected with interparticle crushing is not diminished by the above-described difficulties. The invention can also be fundamentally employed in a dynamic and static sifter.
The sifter of the invention is advantageously a dynamic sifter comprising a sifter housing in the lower region of which the entry opening for separating air and the discharge opening for the coarse grits are arranged and in which upper region a rotatably arranged rod basket having turbo elements is arranged that is surrounded by baffle elements arranged and distributed over the circumference thereof. It is precisely in such a turbo air separator that the wear at the sifter rotor, at the baffle elements and at the other inside sifter parts is diminished due to the pre-separation of the coarse grits. This is because the coarse grits in the sifter of the invention are withdrawn from the sifter, separately from the finer grits before they can come into contact with the sifter inserts.
The sifter of the invention is suitable for use with a circulating grinding system comprising a high-pressure roller press and comprising a fine comminution device such as a tube mill, being inserted thereinto in such a way that the sifter discharge for the coarse grits is returned to the product delivery of the roller press and the sifter discharge for the fine grits is returned to the product delivery of the tube mill. In this solution, both the high-pressure roller press having interparticle crushing and the tube mill operate under optimum conditions.
Other advantages, features and objects will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiment thereof in the specification and drawings, in which: